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<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
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  <title>Document</title>
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<body>
  <div>a</div>
  <iframe id="frame" src="http://localhost:4000/b.html" frameborder="0" onload="sayb()"></iframe>
  <script>
    function sayb() {
      let frame = document.getElementById("frame");
      frame.contentWindow.postMessage('b页面你好', 'http://localhost:4000');
      // window.onmessage = function (e) {
      //   console.log(e.data);
      // }
    }
  </script>
</body>

</html>